Sustain can have different meanings based on the context: "His breakfast sustained him throughout the race." (as in supported) "He sustained a blow to the head." (as in received / suffered ) Withstand usually means you survive or resist an attack or some kind of bad experience. "He withstood the blow to his head and went on to win the fight." (as in survive / hold out against) "These monuments have withstood the tests of time." They can both be used in similar contexts but the use and meaning is different.